Tkachuk was the grabbed from behind by Gudas, who immediately grabbed Tkachuk's neck while still wearing his glove, a very obvious attempt to injure the Ottawa Senators forward.
Both Brady Tkachuk and Radko Gudas are known for their rough, sometimes dirty styles of play, as they have both accumulated multiple NHL seasons with over 100 PIMS.
While Gudas having multiple 100 PIM seasons is not surprising, it is extremely rare for a player of Tkachuk's calibre to have this stat as well, at least in today's NHL.
Tkachuk has been dominant during these World Championships, and perhaps Gudas was trying to draw a retaliation penalty from Tkachuk, but the way he handled the situation was very poor.
Regardless of what another player thinks of his competitors on the ice, Gudas' actions are unforgiveable, as injuring another player on purpose is unacceptable, not just in the NHL, but in any professional league or international tournament.